1. Field of the Invention
The present invention relates to process for the production of Nb3Al extra-fine multifilamentary superconducting wire. More particularly, the invention concerns a process for the production of Nb3Al extra-fine multifilamentary superconducting wire which can produce simply and inexpensively high-performance Nb3Al extra-fine multifilamentary superconducting wire having three greatly-improved critical values, Tc, Hc2 and Jc, without the addition of third elements such as Ge, Si and Cu, the process being promising as a practically applied technology.
2. Description of the Related Art
A process for producing Nb3Al extra-fine multifilamentary wire has been proposed in which a rapid heating and quenching treatment is applied to a composite wire of Nb and Al to form an Nb-25 at % Al supersaturated bcc alloy solid solution phase, and then an additional heat treatment at 700-900xc2x0 C. is conducted to transform the Nbxe2x80x94Al supersaturated bcc alloy solid solution phase to an Al5.Nb3Al phase.
The Nb3Al extra-fine multifilamentary wires produced by this process have three critical values, Tc, Hc2 and Jc, higher than those of the Nb3Al wires by the conventional diffusion process and, therefore, have much hope as a next practically used wire for strong magnetic fields. Although the maximum magnetic field generated by superconducting magnets using conventional metallic superconducting wires was 21.7 T, it is expected that the use of the aforementioned Nb3Al extra-fine multifilamentary wires will achieve the upper limit of magnetic field generated as high as 21 T at 4.2K and 24 T at 1.8K.
On the other hand, the improvement of performances of Nb3Al extra-fine multifilamentary wires is under studying.
For example, it has been found that the addition of Ge or Si to Nb3Al results in the direct formation of Al5 phase after rapid heating and quenching treatment, greatly enhancing Tc and Hc2.
In this case, however, it has been pointed out that a large amount of impurities are also formed, Jc becomes not so high and the addition of Ge and Si greatly reduces workability of Nb/Al wires. The production of long-length wires requires enough research.
On the other hand, it has been also found that the addition of Cu results in the direct formation of the Al5 phase after a rapid heating and quenching treatment, improving characteristics. Tc reaches 18.2 T and Hc2 reaches about 29 T. Furthermore, Jc is also a high value in strong magnetic fields.
For practical applications of this technology, however, there are problems in workability or the like.
The invention according to this application is created in light of such circumstances and intends to provide a process for the production of an Nb3Al extra-fine multifilamentary superconducting wire which can produce simply and inexpensively a high-performance Nb3Al extra-fine multifilamentary superconducting wire having three greatly-improved critical values, Tc, Hc2 and Jc, without the addition of third elements such as Ge, Si and Cu, the process being promising as a practically applied technology.
To solve the above problems, the invention according to this application provides, in a first aspect, a process for the production of an Nb3Al extra-fine multifilamentary superconducting wire, the process comprising the steps of: applying a first rapid heating and quenching treatment to an Nb/Al composite wire having an atomic ratio of Al to Nb from 1:2.5 to 1:3.5 and having extra-fine multifilamentary structure to form a BCC alloy phase comprising Nb with Al supersaturatedly dissolved therein wherein the first treatment comprises heating the composite wire up to a temperature not lower than 1900xc2x0 C. within two seconds and then introducing it into a molten metal at a temperature not higher than 400xc2x0 C. to rapidly quench it; applying a second rapid heating and quenching treatment to form an Al5 pahse-Nb3Al compound having a low degree of crystalline order but having a composition near the stoichiometric one wherein the second treatment comprises heating the wire up to a temperature not lower than 1500xc2x0 C. within two seconds and then introducing it into a molten metal at a temperature not higher than 400xc2x0 C.; and applying, after the second rapid heating and quenching treatment, an additional heat treatment at 600-850xc2x0 C. to the composite wire to improve the degree of crystalline order of the Al5 phase.Nb3Al compound.
The invention according to this application also provides, in a second aspect, the process further comprising a step, conducted before or after the additional heat treatment, of coating the composite wire with Cu. The invention provides, in a third aspect, one of the processes further comprising a step, conducted before the first rapid heating and quenching treatment, of compounding Cu or Ag for stabilizing in a state where a diffusion barrier is incorporated. The invention also provides, in a fourth aspect, one of the processes further comprising a step, conducted between the first rapid heating and quenching treatment and the second rapid heating and quenching treatment, of cold working at reduction of area of 70% or less.